Rankine cycle apparatus have been known as systems for converting heat energy into mechanical work. The Rankine cycle apparatus include a structure for circulating water as a working medium, in the liquid and gaseous phases within a sealed piping system forming a circulation system in the apparatus. Generally, the Rankine cycle apparatus include a water supplying pump unit, an evaporator, an expander, a condenser, and pipes connecting between these components to provide circulation circuitry.
FIG. 14 is a schematic block diagram of a general setup of a conventionally-known Rankine cycle apparatus (e.g., vehicle-mounted Rankine cycle apparatus) and certain details of a condenser employed in the Rankine cycle apparatus. The Rankine cycle apparatus of FIG. 14 includes a water supplying pump unit 110, an evaporator 111, an expander 107, and the condenser 100. These components 110, 111, 107 and 100 are connected via pipes 108 and 115, to provide circulation circuitry in the apparatus.
Water (liquid-phase working medium), which is supplied, a predetermined amount per minute, by the water supplying pump unit 110 via the pipe 115, is imparted with heat by the evaporator 111 to turn into water vapor (gaseous-phase working medium). The vapor is delivered through the next pipe 115 to the expander 107 that expands the water vapor. Mechanical device (not shown) is driven through the vapor expansion by the expander 107 so as to perform desired mechanical work.
Then, the expanded water vapor is delivered through the pipe 108 to the condenser 100, where the vapor is converted from the vapor phase back to the water phase. After that, the water is returned through the pipe 115 to the water supplying pump unit 110, from which the water is supplied again for repetition of the above actions. The evaporator 111 is constructed to receive heat from an exhaust pipe extending from the exhaust port of the engine of the vehicle. Among various literatures and documents showing structural examples of the Rankine cycle apparatus is Japanese Patent Application Laid-open Publication No. 2002-115504.
The following paragraphs detail a structure and behavior of the condenser 100 in the conventional vehicle-mounted Rankine cycle apparatus shown in FIG. 14.
The condenser 100 includes a vapor introducing chamber 101, a water collecting chamber 102, and a multiplicity of cooling pipes 103 vertically interconnecting the two chambers 101 and 102. In the figure, only one of the cooling pipes 103 is shown in an exaggerative manner. Substantial upper half of the interior of each of the cooling pipes 103 is a vapor (gaseous-phase) portion 104, while a substantial lower half of the interior of the cooling pipe 103 is a water (liquid-phase) portion 105. In the vapor portion 104, most of the working medium introduced via the vapor introducing chamber 101 to the cooling pipe 103 is in the gaseous phase, while, in the water portion 105, most of the working medium flowing through the cooling pipe 103 is kept in the liquid (condensed water) phase. Boundary between the vapor 104 and the water 105 (i.e., gas-liquid interface) is a liquid level position 112.
One cooling fan 106 is disposed behind the cooling pipes 103 (to the right of the cooling pipes 103 in FIG. 14). The cooling fan 106 is surrounded by a cylindrical shroud 106a. Normally, operation of the cooling fan 106 is controlled by an electronic control unit on the basis of a water temperature at an outlet port of the condenser 100. The single cooling fan 106 sends air to the entire region, from top to bottom, of all of the cooling pipes 103 to simultaneously cool the cooling pipes 103.
The condenser 100 operates as follows during operation of the Rankine cycle apparatus. Water vapor of a relatively low temperature, discharged from the expander 107 with a reduced temperature and pressure, is sent into the vapor introducing chamber 101 of the condenser 100 via the low-pressure vapor pipe 108 and then directed into the cooling pipes 103. Cooling air 109 drawn into the cooling fan 106 is sent to the condenser 100.
Strong cooling air is applied by the cooling fan 106 to the upstream vapor portion 104 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where a mixture of the vapor and water exists, and thus latent heat emitted when the vapor liquefies can be recovered effectively by the cooling air. Cooling air is also applied by the cooling fan 106 to the downstream water portion 105 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where substantially only the water exists. Water condensed within the cooling pipes 103 of the condenser 100, is collected into the water collecting chamber 102 and then supplied by the water supplying pump unit 110 to the evaporator 111 in a pressurized condition as noted above.
In FIG. 14, reference numeral 116 represents a surface area of a condensing heat transmission portion, and 117 represents a surface area of a heat transmission portion of the condensed water. The surface areas 116 and 117 of the heat transmission portions and the liquid level position 112 have the following relationship.
The conventional Rankine cycle apparatus 100 inherently has the characteristic that the liquid fluid position 112 varies. Namely, because the engine output varies in response to traveling start/stop and transient traveling velocity variation of the vehicle, the amount of water supply to the evaporator 111 also varies, in response to which the liquid level position 112 within the condenser 100 varies. Namely, in the condenser 100, the liquid level position 112 rises when the amount of the vapor flowing into the condenser 100 (i.e., inflow amount of the vapor) is greater than the amount of the condensed water discharged from the condenser 100 (i.e., discharge amount of the condensed water), but lowers when the inflow amount of the vapor is smaller than the discharge amount of the condensed water. In this way, the vapor-occupied portion (104) in the cooling pipes 103 of the condenser 100 increases or decreases. Because the condensed water (in the portion 105) is discharged from the water supplying pump unit 110 subjected to predetermined flow rate control, a pressure from an outlet port 113 of the expander 107 to an inlet port 114 of the water supplying pump unit 110 is determined by a pressure within the condenser 100. The pressure within the condenser 100 is determined by an amount of condensing heat exchange caused by cooling of the vapor portion (104) of the condenser, and the amount of condensing heat exchange is determined by a flow rate of the medium to be cooled and a surface area of the condensing heat transmission portion 116. Thus, if the portion occupied with the vapor increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 116 of the condensing heat transmission portion increases or decreases and so the pressure within the condenser 100 and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer.
Similarly, the temperature of the condensed water at the outlet port of the condenser 100 is determined by an amount of heat exchange caused by cooling of the water portion (105) of the condenser, and the amount of the heat exchange of the condensed water is determined by the flow rate of the medium to be cooled and a surface area 117 of a heat transmission portion of the condensed water. Thus, if the portion occupied with the condensed water (105) increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 117 of the heat transmission of the condensed water portion increases or decreases and so the temperature of the condensed water and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer.
Fundamentally, each of the known Rankine cycle apparatus is constructed on the assumption that it is operated in a closed circulation system. Namely, the circulation system, through which the working medium circulates, is ideally a hermetically-sealed closed system. Thus, it is desirable that the working medium circulate in the circulation system without leakage from the circulation system. In reality, however, the working medium tends to be discharged out of the system for the following two reasons. First, pressurization is effected separately between the water supplying pump unit 110, the evaporator 111 and the expander 107, performing primary functions of the Rankine cycle apparatus, during startup at low temperature, which results in a loss of pressure balance between these components. Thus, vapor would leak more or less through sealed portions of the expander 107 etc. Second, the working medium having completed its predetermined work in the expander 107 is converted from vapor into water and then delivered from the condenser 100 to a water tank in preparation for a next work cycle. However, the water is sometimes intentionally discharged out of the circulation system in order to keep a predetermined liquid level position of the working medium within the condenser 100.
Actually, in the Rankine cycle apparatus, it is necessary to maintain a constant total amount of the working medium in the entire circulation system. For this purpose, the working medium, let out of the circulation system due to the leakage and intentional discharge, has to be returned or re-delivered into the circulation system in one way or another in order to constantly secure the predetermined total amount of the working medium.
Two major ways are conceivable to return the leaked working medium to the circulation system. One of the ways is to return the leaked working medium from each of a plurality of leak circuits directly to a main circuit. The other way is to temporarily receive, in an open tank, the leaked working medium from the plurality of leak circuits and then return the temporarily-received leaked working medium or water to the main circuit. With the first-mentioned way of returning the leaked working medium to the circulation system, it is necessary to insert a separate pump in each of the plurality of leak circuits and control operation of the pumps relatively precisely in accordance with a leaked amount of the working medium, so that the system would unavoidably become complicated in structure. With the second-mentioned way, on the other hand, there is a need to automatically return the temporarily-received water from the open tank to the circulation system in such a manner as to compensate for the leaked amount, and thus some measurement means and control means would be required.
Alternatively, the leaked working medium may be returned manually, in which case the system structure can be simplified significantly. However, when the vehicle having the Rankine cycle apparatus mounted thereon is traveling in particular, it is extremely difficult to manually replenish the working medium during driving operation. Further, for replenishment during travel of the vehicle, it is necessary to secure an amount of the working medium which is equal to the amount of the working medium leaked out of the circulation system. To cover a sufficient traveling distance (or time), a relatively great tank is required although the exact necessary size of the tank depends on driving conditions etc. Therefore, the Rankine cycle apparatus tends to have an increased overall size and hence lack practicality or utility.
The conventionally-known Rankine cycle apparatus generally have the following three challenges of: (1) allowing the working medium, leaked out of the circulation system, to be automatically returned to the circulation system; (2) being constructible into a sufficiently-small size for mounting on a vehicle; and (3) being capable of readily achieving optimization means, without using complicated control etc., for mounting on a vehicle.
JP-A-2003-97222 discloses a technique for use with a Rankine cycle apparatus, where a mixture of expander-lubricating oil and water is supplied to a coalescer-based water separation section for separation of the water from the oil. Here, the oil separated from the water is returned to the expander, while the separated water is returned to circulation circuitry. However, the disclosed Rankine cycle apparatus does not have the function of maintaining a predetermined liquid level position within the condenser as discussed above, and so it is not constructed to intentionally discharge water out of the circulation system and has no open tank for accumulating or storing the thus-discharged water. Therefore, the technique disclosed in the 2003-97222 publication can not be applied as-is to Rankine cycle apparatus provided with an open tank for storing water discharged out of the circulation system. Examples of conventionally-known systems, which are constructed like working medium supply systems for automatically replenishing leaked working medium in Rankine cycle apparatus, include engine cooling systems of automotive vehicles, cooling-medium supply systems of car air-conditioner apparatus, condensing and resting or out-of-operation facilities of steam power plants, etc.
The present invention has been made in view of the foregoing prior art problems and challenges.
Namely, the Rankine cycle apparatus, where the working-medium circulation system is generally constructed as a closed system, cause the working medium to circulate while maintaining a predetermined total amount of the working medium in the entire circulation system. When leakage of the working medium occurs in the circulation system, there arises a need to automatically replenish or re-supply the leaked amount of the working medium to the circulation system, in order to maintain the predetermined total amount and thereby prevent the operation of the Rankine cycle apparatus from being adversely influenced by a decrease in the amount of the working medium within the entire circulation system. Further, in the Rankine cycle apparatus constructed to maintain a predetermined fluid level position within the condenser, arrangements have to be made for replenishing the leaked amount of the working medium at appropriate timing and in a precisely- and accurately-controlled amount. Further, a working-medium replenishing supply system employed in the Rankine cycle apparatus has to be of a simple and small-size structure.
Therefore, there has been a demand for a novel Rankine cycle apparatus which includes a working-medium supply system capable of replenishing the working medium, let out from the circulation system, at appropriate timing and in a precisely- and accurately-controlled amount.
Further, there has been a demand for a novel technique which is suitably applicable to the Rankine cycle apparatus constructed to maintain a predetermined fluid level position within the condenser, and which is capable of replenishing the working medium, discharged out of the circulation system, at appropriate timing and in a precisely- and accurately-controlled amount so as to reliably maintain the predetermined fluid level position.
Furthermore, there has been a demand for a novel Rankine cycle apparatus for mounting on a vehicle, such as an automotive vehicle, which allows the working medium, let out of the circulation system, to be automatically returned to the circulation system, and which has a simplified structure and reduced size.